The present invention relates to a membrane module for hydrogen separation.
Fuel cell systems, in particular those for mobile applications, may be supplied with hydrogen by reforming methanol or hydrocarbons, such as gasoline, or diesel, for example. The product gas, obtained in a reforming process, contains not only hydrogen, but also carbon monoxide, carbon dioxide, and water vapor. For the application in the fuel cell, the carbon monoxide in particular must be removed since this gas acts as a catalyst poison and results in efficiency loss of the fuel cell.
Membranes, which may be made of different materials such as ceramic, glass, polymer, or metal, for example, have been in use for a long time for hydrogen separation. Metal membranes stand out because of their high hydrogen selectivity and high temperature stability; they have, however, comparatively low permeation rates.
In order to achieve a desired permeation rate, a plurality of membrane cells is used, each having a hydrogen-selective membrane; a hydrogen-containing reformate gas flow strikes them either successively (serially) or in parallel. The membrane cells are stacked on top of one another in order to form a compact membrane module.
Membrane modules having a serial flow are described in U.S. Pat. No. 5,498,278 and in U.S. Pat. No. 5,645,626, for example, the subject matter of both patents being incorporated by reference herein.
A membrane module having a parallel flow, is known from international patent publication WO 01/70376 and contains a plurality of flat membrane packs. Each membrane pack contains two membrane assemblies, each being formed of one or two membrane frames acting as a carrier for a hydrogen-selective flat membrane, as well as a support structure between the two membrane assemblies. A plurality of these membrane packs are stacked on top of one another forming a compact stack having flat lateral surfaces, feed spaces for reformate gas being kept open between the individual membrane packs using sandwiched feed frames. At the top and bottom, this stack is held together and locked via flat end plates.
The end plates must absorb the internal gas pressure of the membrane module against the ambient pressure without bending. The end plates must be very solid since the hydrogen-containing reformate gas, obtained in an upstream reforming process from, for example, methanol, gasoline, or diesel and fed to the membrane module, is under a significantly higher pressure than the atmospheric pressure. Like many other components of the membrane module, the end plates are frequently made of metal and are therefore heavy. Their heat capacity is correspondingly high, so that heating up the membrane module to operating temperature requires considerable time and energy. In addition, complete and pressure-resistant welding of all membrane packs among one another is necessary from the design standpoint in order to achieve the required gas tightness. In the case of damage, the membrane module is irreparable since it is impossible to open the weld seam in order to replace a defective membrane.